Extra-hepatic postprandial metabolism of dietary fructose

高糖摄入，特别是果糖摄入，会导致与2型糖尿病（T2D）、心血管疾病（CVD）和非酒精性脂肪肝疾病风险增加相关的脂质谱。导致这些疾病的潜在代谢机制在很大程度上仍然未知。该提案的母体研究R01DK116033侧重于果糖在肝脏中的作用和代谢命运，肝脏历来被认为是处理果糖的主要组织。该建议考虑了我们的初步数据，这些数据支持非肝组织作为果糖代谢位点的主要作用，以及影响餐后脂质谱的载脂蛋白清除因子。我们解决了三个与非肝脏果糖代谢有关的问题，在糖尿病前期和对照组受试者中使用高果糖和低果糖膳食。我们的第一个问题是关于小肠在果糖代谢中的作用；特别是通过新生脂肪生成（DNL）将果糖转化为脂肪，乳糜微粒（CM）中载脂蛋白的组成，即载脂蛋白ii和载脂蛋白ob，以及它们对餐后高脂血症的相对贡献。我们开发了一种免疫亲和方法来分离小肠中脂肪运输的CM。这将使我们第一次能够量化肠道DNL和CM的载脂蛋白组成，而不受肝脏中脂肪的污染，并由极低密度脂蛋白（VLDL）运输。我们的第二个问题是，有多少果糖逃过了肠道和肝脏的代谢，进入循环系统，从而进入其他组织。为此，我们将使用最近发表的双稳定同位素方法来测量血浆果糖水平。此外，这种方法将使我们能够首次探索人类肠道糖异生。对于我们的第三个问题，我们将通过确定完全氧化产生标记的13CO2的果糖量来测量果糖被用作燃料的量。最近的研究表明，氧化是果糖代谢的另一个主要途径。本研究对18名糖尿病前期患者和18名对照组患者口服和静脉注射稳定同位素，并应用一种分离TRL的技术，定量测定CM和VLDL载脂蛋白以及肠道和肝脏DNL，同时测量外周循环中的果糖，估计肠道糖异生和果糖氧化。通过测量DNL、清除因子、循环果糖和果糖氧化，并结合母体研究（肝脏糖异生、TRL-DNL和糖原储存）的测量，我们将对果糖代谢有一个更完整的表征，并回答关于两个不同人群中果糖命运的长期问题。重要的是，我们将证明肝外果糖代谢如何导致T2D和CVD风险。

High sugar consumption, and more specifically fructose intake, leads to lipid profiles associated with increased risk of type 2 diabetes (T2D), cardiovascular disease (CVD), and non-alcoholic fatty liver disease. The underlying metabolic mechanisms leading to these conditions remain largely unknown. The parent study for this proposal, R01DK116033, focuses on the role and metabolic fates of fructose in the liver, which historically was considered the main tissue handling fructose. This proposal takes into account our preliminary data supporting a major role of non-hepatic tissues as sites of fructose metabolism, and of apolipoprotein clearance factors influencing postprandial lipid profiles. We address three questions related to non-hepatic fructose metabolism using high- and low-fructose meals in pre-diabetic and control subjects enrolled in the parent trial. Our first question regards the role of the small intestine in fructose metabolism; specifically the conversion of fructose to fat by de novo lipogenesis (DNL) and the composition of apolipoproteins in chylomicrons (CM), namely apoCIII, and apoB, and their relative contribution to postprandial hyperlipidemia. We developed an immunoaffinity method to isolate CM that transport the fat packaged in the small intestine. This will allow us to quantify, for the first time, intestinal DNL and the apolipoprotein composition of CM without contamination from fat made in the liver and transported by very-low density lipoproteins (VLDL). Our second question addresses the amount of fructose that escapes intestinal and hepatic metabolism and appears in the circulation, thereby becoming available to other tissues. For this aim, we will use a recently published dual stable isotope method to measure plasma fructose levels. Additionally, this method will allow us to explore, for the first time, intestinal gluconeogenesis in humans. For our third question, we will measure how much fructose is used as fuel by determining the amount of fructose that is completely oxidized to produce labeled 13CO2. Recent studies have suggested that oxidation is another major route of fructose metabolism. The administration of oral and intravenous stable isotopes in 18 pre-diabetic and 18 control subjects enrolled in the parent trial and the application of a technique to fractionate TRL allows for the quantification of CM and VLDL apolipoproteins and intestinal- and hepatic- DNL, along with measurements of fructose in the peripheral circulation, estimation of intestinal gluconeogenesis, and fructose oxidation. By measuring DNL, clearance factors, circulating fructose, and fructose oxidation in conjunction with the measurements of the parent study (hepatic gluconeogenesis, TRL-DNL and glycogen storage), we will have a more complete characterization of fructose metabolism and answer longstanding questions regarding the fates of fructose in two distinct populations. Importantly, we will demonstrate how extra-hepatic fructose metabolism contributes to T2D and CVD risk.